Method and device for multiple stage arc-welding

ABSTRACT

The invention relates to a multiple stage method for arc-welding a bolt ( 3 ) to a metal structure ( 1 ), said bolt ( 3 ) having a flange ( 5 ) at the end that is to be welded on. The actual welding process is preceded by a cleaning method that is carried out by means of an arc. Said cleaning method can be advantageously carried out in two or more stages with different effects. It is crucial to control the arc and the distance between the bolt ( 3 ) and the structure surface ( 7 ) in such a way that only cleaning takes place, without causing premature welding. While during the first cleaning state, only a small surface can be cleaned intensively, in the second stage, a large surface can be cleaned due to a greater distance between the bolt ( 3 ) and the structure surface ( 7 ). Separation through interruption of the arc and maintenance of the bolt (3) in a stationary position at a distance from the structure surface ( 7 ) must take place between the first and second cleaning stages. The second cleaning stage can lead into the actual welding process continuously however, without interrupting the arc, the bolt ( 3 ) then being permanently connected to the structure ( 1 ). The invention method is particularly suitable for welding bolts to aluminum structures, even where a surface coating is present, e.g. an oxide and/or a zinc layer and/or an oil film and/or a wax layer.

[0001] The invention relates to a method and device for multiple-stagearc welding, whereby a bolt having a flange at the end that is to bewelded on is moved up to a metal structure and welded on.

[0002] Welding a metal bolt to a metal structure by means of arc weldingis a widely used method, which finds application especially in theautomobile industry. There are essentially different welding methodswith corresponding designs of the associated bolts.

[0003] In one welding method, the so-called lifting ignition method, theend of the bolt to be welded on has a flange that on its undersideusually also has a bead of material that may be domed or has the shapeof a blunt cone. In the lifting ignition method, the bead of the bolt isfirst moved up to a structure until an electrical contact is made. Ahigh electric short circuit current from a welding power source is sentthrough the bolt and the structure. The bolt is then again slightlylifted from the structure, whereby an electric arc is produced betweenthe bolt and the structure that fuses the structure in a fusion regionas well as parts of the bead. After a specified period the bolt islowered into the fused mass produced in the fusion region, whereby onsubsequent cooling a very sturdy weld joint is produced between the boltand the structure.

[0004] Another method for arc welding bolts to a metal structure is theso-called tip ignition method. There the end of the bolt to be welded onhas a salient or projecting tip, which is first moved up to thestructure. The tip is dimensioned in such a way that when a weldingcurrent is applied it is suddenly vaporized with formation of a hotplasma, whereby the remaining end to be welded on and the structure arefused. At the same time, the bolt is rapidly lowered into the fusedmass. Such a method is described for example in DE 4,236,527. The greatenergy required for this operation results in a very loud explosivenoise. In addition, the structure generally must be supported on theback of the fusion region, since otherwise the impact of the bolt athigh speed may cause vibrations that have an adverse effect on weldquality. Besides, the quality of the weld is greatly dependent on thequality of the tip.

[0005] The increasingly broader application of bolt welding methods alsoresults in applications that cannot be accomplished particularly well byeither of these two methods. Thus, there are cases in which thestructure in the fusion region is coated or fouled, especially with awax layer or oil film, and cases in which the structure is a material,particularly aluminum, having an oxide layer. Coatings deleterious tothe welding operation, such as for example hot galvanizing, are alsoencountered. Combinations of these cases likewise occur.

[0006] It is well known that two stages may be provided in bolt welding,the first stage acting to establish the surface condition and/or being acleaning stage and the second stage producing the weld joint. However, areproducibly great cleaned surface cannot always be produced in a singlecleaning stage, since in variable surface conditions, the first ignitionof an electric arc plainly leads to results that are hard to reproduce.

[0007] For such cases, the object of the present invention is to makeavailable a generic method and a device for arc welding a bolt to ametal structure, which permits high-quality and reproducible weld jointson surfaces that are provided with an oxide layer and/or some othersurface coating, in particular oil, wax or even a zinc layer, where themethod should also work under acceptable operating conditions and beeconomical.

[0008] A method according to claim 1 and a device according to claim 13serve to accomplish this object. Advantageous embodiments are indicatedin the respective dependent claims.

[0009] As concerns method, the accomplishment consists in that, by meansof an arc generated between the bolt and the structure, the surface ofthe structure in the fusion region is first cleaned in at least twocleaning steps and only then is the metallic connection between the boltand the structure made by customary electric-arc bolt welding

[0010] When the distance of the bolt from the structure and theoperating time of the electric arc are correctly coordinated, the arcmay in particular be used for cleaning the structure surface of an oilfilm or wax layer, as well as of oxides or hot galvanizing. In this way,thorough cleaning of the structure surface can be obtained in at leastthe fusion region of the bolt, so that the subsequent metallicconnection can be reliably obtained by electric-arc bolt welding withoutspecial effort. There the first cleaning step by ignition of ahigh-power but narrowly limited electric arc produces a precise contactarea, which upon renewed lowering of the bolt and its lifting permits ahighly reproducible second cleaning arc that then cleans the entirelater fusion region.

[0011] According to an advantageous development, cleaning of thestructure surface therefore is carried out in two separate successivecleaning stages where control of the entire multiple-stage arc-weldingmethod is effected by moving the bolt in the direction of the structureand away from it, as well as by the time and duration of the electricalconnection of the bolt with the welding current source. Differentaspects of the cleaning operation may be focused on in this two-stagecleaning by means of electric arc.

[0012] In particular, if the method in the first cleaning stage iscarried out according to the specification of claim 3, especiallyintensive cleaning of a relatively small surface is obtained, whichcorresponds approximately to part of the cross-sectional flange area ofthe bolt. In this method stage, especially oil films and oxides aresatisfactorily removed.

[0013] Advantageous operating parameters for this first cleaning stageare indicated in claims 4 to 6.

[0014] After completion of the first cleaning stage the bolt remainsraised from the structure, and the electric arc remains disconnected.Accordingly, the two cleaning stages are completely separated from oneanother, and excessive heating of the bolt and structure is alsoavoided, so that upon renewed lowering the bolt does not encounter amolten surface. Due to the fairly great distance of the bolt from thestructure surface, operation of the method in the second cleaning stageaccording to claim 7 results in an electric arc that cleans a relativelygreat partial region of the structure surface; this region correspondsto approximately the size of the cross-sectional flange area of thebolt. This cleaning stage is especially suitable for also removing waxlayers from the structure surface, where these wax layers may be presentin a thickness of 1 to 10 g/m². In this way, all prerequisites forsubsequent metallic connection by electric-arc bolt welding are present.In the second cleaning stage, the distance of the bolt from thestructure is selected such that a sufficiently great region of thestructure can be cleaned. The most favorable distance therefore dependsapproximately on the size of the later fusion region, i.e., on thediameter of the flange to be welded on.

[0015] Since fusion of the structure surface must take place at thispoint anyway, the second cleaning stage leads smoothly into theoperation of electric-arc bolt welding; i.e., the arc remainsuninterrupted in these two method stages.

[0016] Only by correct operation of the electric arc, necessary in everycase, is it thus possible first to carry out thorough cleaning of thecrucial regions of the structure surface and then to obtain anespecially high-grade metallic connection by the method according to theinvention.

[0017] Claims 9 to 11 indicate the materials with which the methodaccording to the invention can be especially well implemented.

[0018] Performance of one or more additional cleaning stages is possibleand may be of advantage for certain applications.

[0019] According to an especially advantageous refinement of theinvention, before cleaning a characteristic magnitude that isproportional to the degree of fouling of the surfaces to be weldedtogether, by means of which the number of cleaning stages is determined,is first measured. This is used to determine the degree of requiredcleaning and insufficient or excessive cleaning of surfaces is avoided.The electrical surface resistance, which in first approximation isproportional to the fouling of the surfaces, is the obviouscharacteristic magnitude. The surface resistance and hence the requirednumber of cleaning stages is determined by means of a current and/orvoltage measurement between the surfaces to be welded together. In aspecial refinement, the contact voltage between the bolt and thestructure is measured.

[0020] In an advantageous development of the method according to theinvention, cleaning is omitted if the characteristic magnitude fallsbelow a first threshold value, takes place once if the characteristicmagnitude lies between the first threshold value and a second thresholdvalue, and takes place at least twice if the characteristic magnitudelies above the second threshold value. Criteria for the number ofcleaning stages, which may differ depending on the type of weld to beobtained, the nature of the surface, the materials to be welded that areused and other welding-specific parameters, are specified by means of atleast one, preferably two or more, threshold values for thecharacteristic magnitude.

[0021] A device for performance of the method must be suitable formaking available even brief and exact fairly strong current surges; thenecessary welding current for the electric arc must be available shortlythereafter. This requires a welding current source with suitable controlmeans, which may for example contain one or two capacitor batteries. Inaddition, the device must have all typical means for performing the liftignition method, namely a bolt holder, a linear drive for moving thebolt holder and a rapid control capable of controlling the course ofmotion of the bolt and the response of the welding current with greataccuracy, in particular with time accuracies in the range of 2 to 20 ms.

[0022] A linear drive, in particular with an electromagnetic drive, hasproved to be especially suitable for the present task. Such drives, asdescribed for example in DE 4,437,264 A1, permit precise control orregulation of the position and speed of the bolt during the weldingoperation.

[0023] The invention is described in detail by an example, specifically,

[0024]FIG. 1 shows a schematic cross section of an arrangement for boltwelding and

[0025]FIG. 2 shows a diagram of the time-dependent course of the boltdistance s from a structure as well as the corresponding course of thecurrent strength I.

[0026]FIG. 1 shows, in cross section, the arrangement of a bolt abovethe structure to which it is to be welded, as well as the most importantparts of the associated welding device.

[0027] The numeral 1 designates a structure, for example a part of avehicle body. This structure may be of aluminum or an aluminum alloy andhave a fusion region 2, which may be covered by a surface coating 2 a.This may be an oxide, a zinc layer, an oil film or even a wax layer. Abolt 3 to be welded, having a bolt shaft 4 and a flange 5, which at itslower end has a bead 6 of material, is arranged above the structure 1 ona bolt holder 8. The bolt holder 8 can be moved axially, preciselyregulated or controlled, by means of a linear drive 9. Used for thispurpose is a control unit 10, which is connected with the linear drive 9via a control line 13. In addition, the control unit 10 is alsoconnected with a welding current source 11 via a control line 14, thewelding current source 11 supplying the bolt holder 8 with the requiredwelding current via a current line 15. The welding current source 11contains for example a capacitor battery 12.

[0028] For performance of the method according to the invention, thebolt 3 in the bolt holder 8 is lowered far enough to contact thestructure surface 7 electrically in the fusion region 2. A firstdischarge ignites an electric arc, which produces cleaning of thestructure surface on a small area that corresponds approximately to partof the cross sectional flange area of the bolt. However, the connectionto the welding current source 11 is interrupted after 40 ms, and thebolt 3 remains in its position at a distance from the structure surface7 until after complete extinction of the electric arc. Its outage phasemay last approximately 30 ms or longer. In this way, overheating of thebolt and structure surface is avoided, and welding on of the bolt thenext time it is lowered is prevented. The distance of the bolt 3 fromthe structure surface 7 during this first cleaning stage isapproximately 1.0 to 3.0 mm.

[0029] The second cleaning stage is then initiated by the bolt againbeing moved out of its spaced-apart rest position up to the structuresurface 7, until it contacts the latter in the fusion region 2. Now anelectric arc is again ignited, but the bolt is moved away a greaterdistance from the structure surface than before and is held there. Inthis way, cleaning of the structure surface is effected on a greatersurface area than in the first cleaning stage. This time the cleanedarea may exceed the cross sectional flange area of the bolt 3.

[0030] The impurities in the fusion region, labeled 2 a, may be removedby this two-stage cleaning. Here, the first cleaning stage, actingintensively on a relatively small surface area, serves primarily toremove oxide and oil films, while with the second cleaning stage waxlayers in a thickness of 1 to 10 g/m² are also eliminated. In bothcleaning stages, the current strength may be approximately 100 mA.

[0031] After completion of the second cleaning stage the electric arc isnot interrupted. Rather, the bolt is again moved nearer to the structuresurface 7, until it has a distance suitable for initiating the weldingoperation. Thus, the second cleaning stage leads smoothly into theoperation of metallic connection by arc welding. Lastly, the bolt 3 isbrought into contact with the now metallically clean fusion region 2. Atthe end of the welding operation a sturdy weld joint exists betweenstructure 1 and welding bolt 3. The exact time sequence of the twocleaning operations, the cooling times and the welding method requireprecise control of the motions of the bolt holder 8 and the weldingcurrent source 11. A drive with a linear motor, as also used inconventional lift ignition methods, is especially suitable for precisecontrol.

[0032] The operations described are also reflected in FIG. 2, in whichthe time courses of current strength I and the distance s of the bolt 3from the structure 1 are represented. It can be seen that the current Iis turned on while the bolt 3 is still resting on the structure 1. Anelectric arc is ignited upon lifting of the bolt 3, whereby the currentI increases for the first cleaning stage. After the current I is turnedoff, the bolt to be welded remains raised until the surface of thestructure 1 is cooled off. Renewed lowering of the bolt 3, connection ofthe current 1, lifting of the bolt 3 for the second cleaning stage,lowering of the bolt 3 for increase of the welding current and, lastly,lowering of the bolt 3 into the fused mass with simultaneousdisconnection of the current take place.

[0033] The present invention is suitable especially for automatedwelding methods in which a plurality of welding bolts are to be placedon structures, particularly of aluminum or aluminum alloys, with weldjoints of high quality and reproducibility. 1 Structure 2 Fusion region2a Surface coating (oxide, oil, wax) 3 Bolt to be welded 4 Bolt shaft 5Flange 6 Bead of material 7 Structure surface 8 Bolt holder 9 Lineardrive 10 Control unit 11 Welding current source 12 Capacitor battery 13Control line 14 Control line 15 Current line s Distance of bolt fromstructure l Current strength t Time

1. Method for multiple-stage electric-arc welding of bolts, whereby abolt (3) having a flange (5) at the end that is to be welded on, ismoved up to a metal structure (1) and welded on, characterized in that,by means of an arc generated between the bolt (3) and the structure (1),the surface (7) of the structure (1) in the fusion region (2) is firstcleaned in at least two cleaning stages and only then is the metallicconnection between the bolt (3) and the structure (1) made by customaryelectric-arc bolt welding.
 2. Method according to claim 1, characterizedin that cleaning of the structure surface (7) is carried out in twoseparate successive cleaning stages and in that control of the entiremultiple-stage electric-arc welding method is effected by moving thebolt (3) in the direction of the structure (1) and away from it, as wellas by the start and duration of the electrical connection of the bolt(3) with the welding current source (11).
 3. Method according to claim2, characterized by the following course of the first cleaning stage: a)the bolt (3) is moved up to the structure (1) until it contacts thestructure (1) in a fusion region (2), b) the bolt (3), upon contactingthe structure (1) at the latest, is electrically connected with thewelding current source (11) and is moved away to a distance from thestructure surface (7) such that the resulting electric arc producescleaning of the structure surface (7) on a small area that correspondsto part of the cross-sectional flange area, c) the connection to thewelding current source (11) is interrupted, and the bolt (3) remains inits position at a distance from the structure surface (7) until afterextinction of the electric arc.
 4. Method according to claim 3,characterized in that in the first cleaning stage work is performed at acurrent strength of about 100 A and the duration of the first cleaningstage is approximately 40 ms.
 5. Method according to claim 3 or 4,characterized in that the distance between the bolt (3) and thestructure surface (7) in the fusion region (2) during the first cleaningstage is approximately 1.0 to 3.0 mm, in particular is of approximatelythe same order of magnitude as the diameter of the fusion region (2). 6.Method according to any of claims 3 to 5, characterized in that theresting state of the bolt (3) at a distance from the structure surface(7) lasts at least 30 ms after disconnection of the welding currentsource (11).
 7. Method according to any of claims 2 to 6, characterizedby the following course of the second cleaning stage: a) the bolt (3) isagain moved up to the structure (1) from its spaced-apart rest positionforming the conclusion of the first cleaning stage until it contacts thestructure (1) in the fusion region (2), b) the bolt (3), upon contactingthe structure (1) at the latest, is again connected with the weldingcurrent source (11) and is moved away from the structure surface (7) toa distance that is equal to or greater than in the first cleaning stage,resulting in the electric arc that is formed producing cleaning of thestructure surface (7) on a relatively great area that exceeds the sizeof the cross-sectional flange area, c) the distance between the bolt (3)and the structure surface (7) in the fusion region (2) is reduced, withuninterrupted continuation of the electric arc, to a value that issuitable for fusing the structure surface (7), and hence the last methodstage of metallic connection by electric-arc bolt welding is smoothlyinitiated.
 8. Method according to claim 7, characterized in that in thesecond cleaning stage work is performed at a current strength of 100 mAand the duration of the second cleaning stage is 150 ms.
 9. Methodaccording to any of the preceding claims, where the metal structure (1)consists of aluminum or an aluminum alloy.
 10. Method according to anyof the preceding claims, characterized in that the bolt (3) consists ofa material of the same kind as the structure (1).
 11. Method accordingto claim 10, characterized in that the bolt (3) consists of steel andthe structure (1 ) likewise consists of steel, preferably steel providedwith a surface coating.
 12. Method according to any of the precedingclaims, characterized in that three or more cleaning stages take placebefore the actual welding operation.
 13. Method according to any of thepreceding claims, characterized in that before cleaning, acharacteristic magnitude, which is proportional to the degree of foulingof the surfaces to be welded together and by means of which the numberof cleaning stages is determined, is first measured.
 14. Methodaccording to claim 13, characterized in that a contact voltage betweenthe bolt (3) and the structure (1) is measured.
 15. Method according toclaim 13 or 14, characterized in that cleaning is omitted if thecharacteristic magnitude falls below a first threshold value, takesplace once if the characteristic magnitude lies between the firstthreshold value and a second threshold value, takes place at leasttwice, if the characteristic magnitude lies above the second thresholdvalue.
 16. Device for performance of the method according to any of thepreceding claims having a welding current source (11) preferablycontaining two capacitor batteries (12 a, 12 b), a bolt holder (8), alinear drive (9) for moving the bolt holder (8) and a rapid control(10), which is capable of controlling the course of motion and ofwelding current with an accuracy in the range of 2 to 20 ms.
 17. Deviceaccording to claim 13, characterized in that the linear drive comprisesan electromagnetic linear motor.